Frequency converters are being increasingly used for wind energy installations. Modern wind energy installations normally have a variable rotation speed, as a result of which the generator produces alternating current at a different frequency. Frequency conversion is necessary for feeding into a fixed-frequency supply grid (normally 50 Hz). Frequency converters are used for this purpose. In this case, it is preferable to use converters which consist of a generator-side inverter which is electrically connected to the rotor of the generator, a DC voltage intermediate circuit and a grid-side inverter which is electrically connected to the grid. Wind energy installations have normally been operated such that they are disconnected from the grid in the event of grid faults, in particular short circuits. With the increasingly widespread use of wind energy installations and the increase in the installed wind power, the requirement is, however, no longer simply for real current to be fed into the electrical grid, but it is desirable for the wind energy installations to be operated to support the grid voltage. In the case of the doubly-fed asynchronous generators which are preferably used for relatively high power wind energy installations, one problem that arises is that the grid-feeding inverters are designed for only about ⅓ of the electrical power and it is now no longer possible to achieve the additionally required currents in order to ensure grid-supporting characteristics.
DE 10 2007 028 582 A1 has proposed that, for a generator with a full converter, in which the entire electrical power is fed via the converter into the electrical grid, the generator-side inverter should likewise be connected directly to the grid when the generator is no longer generating energy, in order to feed additional reactive power into the grid via the generator-side inverter which is then connected directly to the grid.
This solution has the disadvantage that it can be used only when the generator rotor is not emitting any power.